The present invention relates to heat-resistant steels which exhibit high strength even at high temperatures of 700.degree.-1150.degree. C. and which also exhibit superior formability.
HK 40 steels (25 Cr-20 Ni Heat-Resistant Cast Steels) have been widely used in the chemical industry in high-temperature devices. For example, they have been used as tubes for cracking furnaces of ethylene-manufacturing plants and tubes for reforming furnaces for producing hydrogen gas. However, since such tubes are produced by centrifugal casting, it is rather difficult to manufacture small diameter tubes, thinwalled tubes, and lengthy tubes, and the resulting tubes suffer from poor ductility and toughness.
Alloy 800H (0.08 C-20 Cr-32 Ni-0.4 Ti-0.4 Al) has been known as a material for making forged tubing. However, this alloy does not have a satisfactory high-temperature strength.
Recently, cracking furnaces of ethylene plants are being operated at higher temperatures than in the past so as to increase the yields of the products. Therefore, the materials which constitute cracking furnaces must have greater high-temperature strength than in the past.
There are many new materials for use in centrifugally cast tubing which have a higher level of strength than HK 40 steels. Some examples of these alloys are HP, HP-Nb, HP-Nb,W, and BST. Forged tubing materials which correspond to these new alloys are nickel-based alloys such as Hastelloy X (0.06 -21 Cr-9 Mo-1 Co-bal. Ni), Inconel 617 (0.06 C-21 Cr-8.5 Mo-12 Co-1 Al-bal.Ni), and Inconel 625 (0.04 C-21 Cr-9 Mo-3.5 Nb-bal.Ni). However, since these Ni-based alloys contain a great amount of the very expensive elements Mo and Ni, these alloys have problems with respect to economy and formability.
In order to increase reaction efficiency and perform reactions under stable conditions in various high-temperature apparatuses, there is a need for a forged tubing material which has excellent high-temperature strength and which can be used to manufacture lengthy piping with a small diameter.
Materials for use in cracking furnaces and reforming furnaces must have high-temperature strength and a particularly high creep rupture strength, since such materials are used at extremely high temperatures of about 700.degree.-1150.degree. C. Therefore, a centrifugally cast tube has been used for such purposes because it exhibits satisfactory high-temperature strength and is economical.
However, it is difficult to manufacture a lengthy tube with a thin wall and a small diameter by centrifugal casting. Furthermore, centrifugally cast tubes have unsatisfactory ductility and toughness, although centrifugally cast tubes with a high carbon content (0.4-0.5%) have excellent creep rupture strength. This is because eutectic carbide precipitates along the grain boundaries.
In forged tubes with a high carbon content, such precipitated eutectic carbides are broken during working including forging and extrusion, resulting in a large amount of undissolved carbides remaining in the matrix without in any way improving the creep rupture strength. In other words, it is necessary to carry out a different type of strengthening for forged piping material, since the presence of these eutectic carbides cannot be used for strengthening.
In Japanese Unexamined Patent Application Disclosure No. 23050/1982, the inventors of the present invention proposed a heat-resistant forging steel in which high strength is achieved by utilizing grain boundary-strengthening elements as well as solid solution-strengthening elements. The proposed steel can exhibit greater high-temperature strength than forged tubing material such as Alloy 800H and centrifugally cast tubing material such as HK40. Its creep rupture strength is a maximum of 2.20 kgf/mm.sup.2 at 1000.degree. C. after 1000 hours, and in particular the strength is 1.70 kgf/mm.sup.2 for the steel (0.27 C-0.52 Si-1.16 Mn-24.42 Cr-24.8 Ni-0.48 Ti-0.34 Al-0.0040 B-bal.Fe). In addition, it can also exhibit satisfactory toughness, and it can be used to produce long, thin-walled tubes with a small diameter. However, it is necessary to increase the content of Mo and W to further strengthen the steel, although the formability is degraded by increasing the content of these elements. Therefore, the Ni content must be increased to achieve a stabilized structure and as a result, the alloy is less economical. In the abovedescribed patent publication, there is no reference to the nitrogen content at all.
Japanese Unexamined Patent Application Disclosure No. 21922/1975 discloses steel compositions similar to those mentioned above. In this application, 0.005-0.05% of magnesium is added to further improve high-temperature properties, and there is no mention of the nitrogen content. The resulting creep rupture strength is only at most 4.6 kgf/mm.sup.2 after 10.sup.3 hours and at most 3.0 kgf/mm.sup.2 after 10.sup.4 hours at 900.degree. C. Based on these data it is estimated that the creep rupture time at 1000.degree. C. and 2 kgf/mm.sup.2 is 391 hours (minimum)-2185 hours (maximum). In particular, the creep rupture time is 391 hours (minimum)-966 hours (maximum) for the steel (0.20 C-0.52 Si-1.1 Mn-22.8 Cr-25.1 Ni-0.53 Ti-0.56 Al-0.005 B-0.012 -Mg-bal. Fe).